mitosis and meiosis

Question 1

Homologous chromosomes

Answer 1

Chromosomes that contain the same genes (at same loci)
(Matched in shape and size)

Question 2

Interphase

Answer 2

Growth 1
- cells grow
- organelles divide
- new proteins made

S phase
- DNA replicates

Growth 2
- cell grows
- organelle divides
- proteins such as spindle fibres made

Question 3

Prophase

Answer 3

• chromosomes condense onto histones
• centrioles move to poles, forming spindle fibres
• nuclear envelope breaks down
• chromosomes free in cytoplasm

Chromosomes
- 2 identical sister chromatids
- joined by centromere

Question 4

Metaphase

Answer 4

• chromosomes line up along equator
• centrioles attach to spindle fibres

chromosomes:
- 2 identical chromatids

Question 5

Anaphase

Answer 5

• centromeres divide, separating chromatids
• spindle fibres contract, pulling chromatids to opposite poles

chromosomes
- individual chromatids
- v shaped as being pulled

Question 6

Telophase

Answer 6

• chromosomes reach poles
• nuclear envelope forms around each group
• cytokinesis finishes

Question 7

results of mitosis

Answer 7

2 genetical identical daughter cells
(to eachother and parent cells)
diploid (2 of each chromosome, one from each parent)

Question 8

meiosis

Answer 8

before: DNA replicates = 2 copies of each chromosome
DNA condenses = double armed chromosomes - made of 2 chromatids (joined by centromere)

meiosis I
- chromosomes arrange into homologous pairs, line up along equator
- homologous pairs separated - halves chromosome number (centromeres do not divide)
- nuclear envelope reforms and cytokensis happens
= 2 haploid cells

meiosis II
(no DNA replication as no interphase)
- chromosomes line up along equators
- centromeres divide, chromatids pulled to opposite poles
= 4 haploid gametes

Question 9

results of meiosis

Answer 9

4 genetically different cells
(to eachother and parent cell)
haploid (half chromosome number)

Question 10

how does meiosis result in genetic variation? (2)

Answer 10

crossing over
- homologous pairs come together
- chromosomes twist around each other and bits of chromatid swap over
= same genes but different combination of alleles

independent segregation of chromosomes
- pairs separated in meiosis I
- alignment of pairs along equator random
- so random which chromosome ends up in each daughter cell
= daughter cells have different combinations of chromosomes

Question 11

what is a chromosome mutation?

Answer 11

mutation caused by error in meiosis
(eg one cell may have multiple chromosome and the other none)

can lead to inherited conditions as error present in gametes (passed on)

all DNA in organism has same mutation as occurs in gametes (all cells made from gametes = genetically identical)

eg non-disjunction - chromosomes fail to separate properly
(in chromsome 21 = Down syndrome)

Question 12

what is a mutation? (+ types)

Answer 12

change in the base sequence of DNA
- alters amino acid sequence and therefore protein made = not functional

substitution
deletion
addition
duplication
translocation

Question 13

why do some mutations not affect the amino acid sequence?

Answer 13

degenerate nature
- substitutions don’t
different triplet but still codes for same amino acid

mutations that change the number of bases do have an affect
- causes a frame shift, subsequent triplet affected and read differently

Question 14

what is a mutagenic agent?

Answer 14

something that increases the rate of mutation

eg UV light, ionising radiation, chemicals

(mutations usually occur spontaneously during replication)

Question 15

how does random fertilisation result in genetic variation?

Answer 15

in fertilisation, random which egg fuses with which sperm

= random combination of alleles in zygote

Question 16

importance of meiosis in life cycle of organisms

Answer 16

• allows organisms to make haploid gametes - fuse in fertilisation
• prevents doubling of chromosome numbers (as haploid when fuse) so maintains number across generations
• allows variation as genetically different

Question 17

why is variation important in organisms?

Answer 17

increases likelihood of survival
some individuals may have more beneficial alleles than others

Question 18

sources of variation

Answer 18

mutations (DNA or chromosome)
random fertilisation

during meiosis:
independent segregation
crossing over

Question 19

why is it important that one chromosome from each homologous pair separates?

Answer 19

so it produces haploid cells
so at fertilisation = diploid zygote with full set of chromosomes

Question 20

how to calculate number of combinations of chromosomes after meiosis (before fertilisation)

Answer 20

2 to the power of n
n = number of homologous pairs

Question 21

how to calculate number of chromosome combinations after fertilisation

Answer 21

(2 to the power of n) squared
n = haploid number